Resolved questionsJ'ai des problèmes sur la liaison.

I'm using a lot of interactive language sets and books but some of them are making liaison or aren't. For example in a book it was written that the pronunciation of "Je suis un élève." is "Je-suis-z.un-élève." but in the set it was pronunciated as "Je-suis-un-élève". Why? Is there a rule where must I make liaison?